Imagine, for just one minute, that vampires, witches and so on really do exist. Where would they go to meet each other? What sort of jobs would they do? In her day-job, Deborah Harkness is an academic historian of science; her novel started, she says, the day she asked herself that question. The resulting opus is 600 pages long, the hit of the 2009 Frankfurt book fair, the first volume in a projected trilogy; I don't think we need even mention Dan Brown or Stephenie Meyer, or the entire walls of our suffering local libraries given over to "urban fantasy" and "dark romance".

It probably is worth noting, however, that as a historian, Harkness specialises in the 17th century, the time when, as her novel puts it, "astrology and witch-hunts yielded to Newton and universal laws"; and that she decided, in answer to her own question, that nowadays vampires and witches would probably work, like her, as academics. Vampires would stick to science – the long hours in chilly labs would suit them. Witches would do well in the humanities. It's a neat concept, and easy to see why the publishers were hooked.

The heroine of A Discovery of Witches (DiscoWitch?) is an American called Diana Bishop, over in Oxford to research alchemical manuscripts in the Bodleian library. She calls up a book from the collection of Elias Ashmole; the illustrations are peculiar. Three pages are missing. Words shimmer across the page. Ashmole 782, Diana realises, is a palimpsest, loaded with occult messages protected by magic spells; she touches it, and the book "lets out a small sigh". Could this be because Diana herself is a witch, albeit trying to live in denial of her heritage? (Which is why, she says, she studies alchemy: "The search for a rational order in nature rather than a supernatural one mirrored my own efforts to stay away from what was hidden," etc.)

But anyway, Diana is being watched. The library is crawling with readers who look human but are in fact vampires; the scrutiny of one in particular is bothersome. He's "tall", with "broad shoulders" and "eyes as black as night". His name is Matthew Clairmont, he's a biochemist and a neuroscientist, a Fellow of the Royal Society, and a Fellow of All Souls. His jerseys are grey cashmere, his shoes alone "cost more than the average academic's entire wardrobe". The mysterious book, Matthew thinks, contains a powerful secret, something to do with witches and vampires and junk DNA. Diana may have stumbled on the missing link between magic and Darwinian evolution; in which case, her life is in deadly danger . . .

As will be obvious by now, this is a very silly novel. Characters and relationships are stereotyped. The historical background is a total pudding. The prose is terrible. And yet, the ideas have just enough suction, somehow, to present an undemanding reader with some nice frissons. I liked, for example, the way Diana tries to sublimate her magic powers in running and rowing and doing yoga – at a mixed vampire-witch-daemonic yoga class, participants struggle not to levitate during their vinyasas. And I liked the way Matthew and Diana smell to each other like Jo Malone candles: Diana is "horehound, frankincense, lady's mantle", Matthew is "cinnamon and clove".

From Dracula to Twilight, stories with vampires in them are always in some way about the middle-class fear and envy of the decadent but mysteriously powerful old rich. This happens too in DiscoWitch, except that fear and envy get conflated into aspirational awe. Matthew's a wine buff, with his own cellar in the All Souls basement; he seduces Diana with Château d'Yquem, just like Hannibal Lecter did Clarice. His diet is posh-paleolithic: carpaccio and oysters, "an enormous salad" composed of "every expensive green known to mankind". The family home is an Occitan fortress. "Dieu!" he exclaims, where a lesser being might say OMG.

With her academic hat on, Harkness is the author of two non-fiction books, on Elizabethan London and John Dee. Fair play to her, I guess, that she's found such a thrifty way of reusing all that material; and interesting that she should hit on doing academics as superhuman beings at just the moment the prospect of a university education slips further and further from people's reach. Not, I'd imagine, that Harkness intended such a message. But think about it: a history professor dropping scholarly research for toshy fantasy. Some actions really do speak louder than words.